Exploration of gas fields can involve discovery of wells that contain significant quantities of hydrogen sulfide and other organic and inorganic sulfur compounds. Oil, natural gas, and water with a high concentration of sulfur compounds such as hydrogen sulfide and sulfur dioxide are referred to as “sour.” Hydrogen sulfide is a colorless, toxic, flammable gas that is responsible for the foul odor of rotten eggs. It often results when bacteria break down organic matter in the absence of oxygen, such as in swamps, and sewers alongside the process of anaerobic digestion. It also occurs in volcanic gases, natural gas and some well waters. Sour oil and sour water are not only undesirable as sour products are economically useful, they can be extremely toxic and deadly because high levels of sulfur and sulfur byproducts. For example, hydrogen sulfide is a highly toxic and extremely deadly gas. The industry considers oil or water containing 100 parts per million (“ppm”) (0.01%) sulfur sour oil and sour water. Although this is the minimum level, oil wells and water can contain higher amounts. Oil and water can contain hydrogen sulfide up to 300,000 ppm (30%) at the immediate gas/liquid interphase, the vapor space in a tank or container, and the atmosphere surrounding a spill. At higher concentrations, hydrogen sulfide is toxic and deadly.
As used herein, the term “sour oil” refers to oil containing levels of hydrogen sulfide in an amount greater than 100 ppm (0.01%). Sour oil can also mean oil containing 0.5% or more sulfur by weight. The term “sour water” refers to water containing hydrogen sulfide in an amount greater than 100 ppm (0.01%). The terms “sweet,” “sweetened,” and/or “sweetening” mean a product that has low levels of hydrogen sulfide, has had hydrogen sulfide removed, or the process of removing hydrogen sulfide. The term “stripping” means removing hydrogen sulfide from water and/or oil. The terms “acceptable limits” or “acceptable amounts” or “acceptable levels” refer to the maximum amount of hydrogen sulfide allowed according to any of the pertinent regulations. For example, the Environmental Protection Agency (“EPA”) has certain regulations regarding the concentration of hydrogen sulfide that may be released into the environment. Furthermore, the Occupational Safety and Health Administration (“OSHA”) provides certain regulations on the amount of hydrogen sulfide one may be exposed to without being considered a health hazard. There may be other regulations that apply, such as state regulations. The terms “acceptable limits” or “acceptable amounts” or “acceptable levels” can also refer to the maximum amount of hydrogen sulfide allowed in oil and/or water in order for a facility to accept the materials.
Exploratory and developmental wells with high concentrations of hydrogen sulfide, far away from hydrogen sulfide removal facilities present a problem of transporting the sour water and sour oil. Both liquids must be transported by truck, sometimes long distances over public and private roads. In most cases, sour water, which is dangerous to transport, will also not be accepted by most re-injection facilities if it contains more than a trace amount of hydrogen sulfide.
Similarly, sour oil, which is also dangerous to transport, will not be accepted by most refineries or pipeline hubs, if it contains more than a trace of hydrogen sulfide. If one finds a facility willing to accept liquids with a high concentration of hydrogen sulfide, odds are they are hundreds of miles away from the exploratory well. A truck accident or a simple leak could endanger the transportation crew, as well as the public.
Raw or unprocessed crude oil is not generally useful in industrial applications, although “light, sweet” (low viscosity, low sulfur) crude oil has been used directly as a burner fuel to produce steam for the propulsion of seagoing vessels. The lighter elements, however, form explosive vapors in the fuel tanks and are therefore hazardous. Instead, the hundreds of different hydrocarbon molecules in crude oil are separated in a refinery into components which can be used as fuels, lubricants, and as feedstocks in petrochemical processes that manufacture such products as plastics, detergents, solvents, elastomers and fibers such as nylon and polyesters.
Petroleum fossil fuels are burned in internal combustion engines to provide power for ships, automobiles, aircraft engines, lawn mowers, chainsaws, and other machines. Different boiling points allow the hydrocarbons to be separated by distillation. Since the lighter liquid products are in great demand for use in internal combustion engines, a modern refinery will convert heavy hydrocarbons and lighter gaseous elements into these higher value products.
Oil can be used in a variety of ways because it contains hydrocarbons of varying molecular masses, forms and lengths such as paraffins, aromatics, naphthenes (or cycloalkanes), alkenes, dienes, and alkynes. While the molecules in crude oil include different atoms such as sulfur and nitrogen, the hydrocarbons are the most common form of molecules, which are molecules of varying lengths and complexity made of hydrogen and carbon atoms, and a small number of oxygen atoms. The differences in the structure of these molecules account for their varying physical and chemical properties, and it is this variety that makes crude oil useful in a broad range of several applications.
Once separated and purified of any contaminants and impurities, the fuel or lubricant can be sold without further processing. Smaller molecules such as isobutane and propylene or butylenes can be recombined to meet specific octane requirements by processes such as alkylation, or more commonly, dimerization. The octane grade of gasoline can also be improved by catalytic reforming, which involves removing hydrogen from hydrocarbons producing compounds with higher octane ratings such as aromatics. Intermediate products such as gasoils can even be reprocessed to break a heavy, long-chained oil into a lighter short-chained one, by various forms of cracking such as fluid catalytic cracking, thermal cracking, and hydrocracking. The final step in gasoline production is the blending of fuels with different octane ratings, vapor pressures, and other properties to meet product specifications. Another method for reprocessing and upgrading these intermediate products (residual oils) uses a devolatilization process to separate usable oil from the waste asphaltene material.
Oil refineries are large scale plants, processing about a hundred thousand to several hundred thousand barrels of crude oil a day. Because of the high capacity, many of the units operate continuously, as opposed to processing in batches, at steady state or nearly steady state for months to years. The high capacity also makes process optimization and advanced process control very desirable.
Many steps may be involved in the refining of crude oil to produce desired products. At least two steps which are usually involved in refining of crude oil are fractional distillation and catalytic cracking. Typically, a crude oil feed is first provided to a crude tower. The crude oil will have been preheated and/or heat is provided to the crude tower by heating fluids such as steam. Lighter components of the crude oil are removed from upper portions of the crude tower while heavier components are removed from lower portions of the crude tower.
The heavy fraction, which is generally referred to as gas oil, is typically provided to a catalytic cracking unit which is generally referred to as the gas oil cracker. The gas oil is cracked to produce lighter, more valuable components in the catalytic cracking unit.
In the past, it has been common to dispose of components of the crude oil which are heavier than the gas oil and which were considered very low value products. However, as it has become necessary to process heavier crudes, it has become more economically desirable to process the components of crude oil which are heavier than gas oil.
It is well known that crude oil may contain components which make processing difficult. As an example, crude oil will generally contain metals such as vanadium, nickel and iron. Such metals will tend to concentrate in the heavier fractions such as the topped crude and residuum. The presence of the metals makes further processing of these heavier fractions difficult since the metals generally act as poisons for catalysts employed in processes such as catalytic cracking.
The presence of other components such as sulfur and nitrogen is also considered detrimental to the processability of the hydrocarbon-containing feed stream. Again, sulfur and nitrogen will tend to concentrate in the heavier fractions. Also, the heavier fractions may contain components (referred to as Ramsbottom carbon residue) which are easily converted to coke in processes such as catalytic cracking.
Processes used to remove components such as metals, sulfur, nitrogen and Ramsbottom carbon residue are often referred to as hydrofining processes (one or all of the described removals may be accomplished in a hydrofining process). Hydrofining processes are used in many refineries to facilitate the processing of heavy fractions of the crude oil such as topped crude and residuum.
In addition to removing undesired components, a hydrofining process will often reduce the amount of heavies in the feedstock to the hydrofining process. This reduction results in the production of lighter components. Typically, when a hydrofining process is used in the refining of crude oil, the gas oil components withdrawn from the hydrofining process are provided to the catalytic cracking unit utilized to crack the gas oil withdrawn from the crude tower. Heavy fractions from the hydrofining unit are typically provided to a second catalytic cracker which is generally referred to as the heavy oil cracker.
In any process for refining crude oil, including processes where hydrofining is practiced, it is desirable to produce a product mix having the highest possible value. High value is determined by determining the amount of each product produced from a barrel of crude oil. The economic value of each product is then determined and a summation gives the value of the product mix. Even very small increases in the value of the product mix are extremely desirable because of the very large volumes of crude oil typically processed in a refinery and also because of the highly competitive of nature of the crude oil refining business.
Traditionally, crude oils are first distilled and then processed further as separate fractions. Conventionally, distillation is initially carried out under atmospheric pressure to produce various distillate fractions including naphtha and middle distillates, as well as an atmospheric residuum or “long” residuum which is then subjected to further distillation under vacuum to produce additional quantities of distillate material together with a vacuum residuum or “short” residuum. This processing scheme which initially separates the components of the crude according to their boiling points has conventionally been regarded as satisfactory because it enables the processing steps which follow the fractionation to be formulated according to the requirements of the individual fractions which vary not only according to their distillation characteristics but also in their chemical compositions.
Crude oil is generally associated with significant quantities of hydrogen sulfide and contains various other organic and inorganic sulfur compounds. Natural fossil fuels, such as crude oil and natural gas, that contain a substantial concentration of sulfur compounds, such as hydrogen sulfide and sulfur dioxide, are referred to as ‘sour.’ Sulfur compounds may evolve from fossil fuels over time and the evolution of these compounds produces significant environmental and safety issues. Emissions of various sulfur compounds, including hydrogen sulfide and sulfur dioxide are regulated. Due to enhanced regulations and restrictions, it is desirable to remove sulfur compounds from crude oil.
Exploratory and developmental wells with high concentrations of hydrogen sulfide, far away from hydrogen sulfide removal facilities present a problem of transporting the sour water and sour oil. Both liquids must be transported by truck, sometimes long distances over public and private roads. In most cases, sour water, which is dangerous to transport, will also not be accepted by most re-injection facilities if it contains more than a trace amount of hydrogen sulfide.
Similarly, sour oil, which is also dangerous to transport, will not be accepted by most refineries or pipeline hubs, if it contains more than a trace of hydrogen sulfide. If one finds a facility willing to accept liquids with a high concentration of hydrogen sulfide, it may be hundreds of miles away from the exploratory well. A truck accident or a simple leak could endanger the transportation crew, as well as the public.
There are other problems downstream in the transportation of sour oil as well. For example, transport from the exploratory well to a treatment site is usually only the first step in the process. The oil typically has an end destination, whether it is another refinery, a distributor, or a consumer. One example can be seen in transportation of oil that is obtained through a fracturing or “fracking” process. Oil extracted through the fracking process typically is sweet and contains little hydrogen sulfide. This oil has to be transported from the site to its end destination. The transportation can be hindered, however, if there is an upstream contamination of sweet oil with hydrogen sulfide of the shipping vessels or oils with different grades are mixed for shipping.
Rail shipment of crude oil has become an option for moving oil out of high production areas with little pipeline access. The shipping industry is adversely affected by having to address the shipping of hydrogen sulfide. The solution to rail safety issues are typically unanticipated costs, including rail car investments or new safety protocols to address the shipping of sour oil.
There is an ever-increasing shortage of naturally-occurring low sulfur crude oil. With the increasing emphasis on pollution control and the resulting demand for low sulfur content petroleum crude oil, a need for the economical production of sulfur-reduced crude has arisen.
Besides meeting enhanced regulations and restrictions, removal of sulfur from crude oil is desirable for other reasons. Not only does the evolution of sulfur compounds from crude oil produce significant environmental and safety issues, these compounds may also attack metal components of the oil well, as well as pipelines and storage tanks and downstream refinery apparatus. This attack causes corrosion and/or brittleness of the metal components. Additionally, in a refinery, downstream processes may utilize catalysts which are sensitive to the presence of sulfur.
In conventional oil refineries, sulfur is generally removed after the crude oil has been fractionated. Sulfur removal typically comprises utilization of various desulfurization processes, often requiring extreme operating conditions, and incorporation of expensive equipment, often associated with high maintenance costs.
Accordingly, there is a need in industry for systems and processes of removing sulfur from crude oil. Desirably, the system and method allow sweetening of crude oil proximal the removal of the oil from the earth.
Certain embodiments of the invention provide a system and a method for the removal of hydrogen sulfide from crude oil streams, such as sour oil streams. In some embodiments, the removal of hydrogen sulfide from sour water streams is additionally or alternatively provided. The method and system for the removal of hydrogen sulfide from a liquid crude oil stream is of lower-cost and of reduced environmental impact than traditional means. In some embodiments, a system and method to sweeten sour oil and water without a need to use hydrocarbons or other catalysts is provided. This is especially useful in the exploratory gas industry when access to traditional methods used to sweeten oil and water are not readily available and could be many miles away. Certain embodiments include a system and a method that comprise collecting the sour oil in a container, maintaining the sour oil in an air-free environment, adding water, and agitating the mixture. Other embodiments of the present invention include using sour water to remove hydrogen sulfide from sour oil.